Effects of Ge and Ag additions on quench sensitivity and mechanical properties of an Al-Zn-Mg-Cu alloy

The effects of Ge and Ag additions on quench sensitivity and mechanical properties of an Al-Zn-Mg-Cu alloy are investigated using tensile testing, hardness testing, differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). Results show that a small addition of Ge significantly reduces quench sensitivity but decreases the ductility of Al-Zn-Mg-Cu alloy. In the Ge-added alloy, some large Mg2Ge particles at grain boundaries are observed, which causes a decrease in the alloy ductility. When Ag is added to Al-Zn-Mg-Cu-Ge alloy, a combination of low quench sensitivity and improved ductility is obtained. It is also found that Ag promotes more uniform decomposition of supersaturated solid solution during aging. As a result, the density of η′ precipitates increases both in the vicinity of grain boundaries and in grain interiors.